Dry ice (DI) is pure, solid-state carbon dioxide (CO.sub.2). Dry ice is normally produced by compacting or pressing crystalline, snow-like CO.sub.2 while it is in the solid state at approximately atmospheric pressure (14.7 psig) and -109.33.degree. F. Although dry ice can also be produced by sub-cooling liquid carbon dioxide (LCO.sub.2), compaction is the preferred method presently used. Mechanical or hydraulic devices are the usual mechanisms used for compaction yielding approximately 92 to 97 lb/cubic foot bulk density of the final product.
In order to produce LCO.sub.2, the essential feed material of the dry ice process, it is necessary to obtain, purify, compress, and liquefy a stream of carbon dioxide gas. This CO.sub.2 gas is usually obtained front an impure by-product stream in a chemical processing plant, a fermentation process, or a natural CO.sub.2 well. Standard industry methods produce bulk LCO.sub.2 that is stored, transported, and distributed at pressurized conditions of approximately 250 psig and -8.degree. F. It is from this basic feed stock that dry ice is usually manufactured.
Dry ice has been traditionally produced and distributed in blocks, typically 10".times.10".times.10". Such blocks weigh about 55 lbs and do not complement the various consumption methods preferred by consumers. Use of block dry ice is cumbersome, labor intensive, and expensive and requires time and expense to crush the blocks in order to reduce them to a reasonable size that be easily handled and used in many applications.
Due to these drawbacks in the use of dry ice in block form, many applications have found the use of dry ice in pellets or nuggets to be more convenient. Such pellets are commonly produced in dry ice pelletizers which are relatively smaller and less expensive than earlier dry ice machines. Pellets are more easily packaged by a manufacturer or subdivided by a consumer into convenient portions which also saves on labor costs.
Unfortunately, no known pelletizer has been available that produces pellets at a rate comparable to existing block presses (nominal capacity of 25 tons/day). To match this capacity requires the use of a plurality of pelletizers which is often not cost effective.
Pellets also have much greater surface for a given quantity of dry ice than blocks. This results in larger sublimation losses during production, storage, and distribution of the dry ice representing a direct economic loss for producers and their customers. Thus, there remains a need for a high volume production pelletizer to reduce the time required to produce a given quantity of dry ice pellets to reduce these sublimation losses.
Dry ice pelletizers produce pellets of dry ice from LCO.sub.2. These dry ice pellets find a vast array of applications, including applications in the processing and preservation of meats and other foods because of the thermal, physical, and chemical properties of dry ice. In certain applications, the dry ice pellets come in intimate contact with the food being processed, such as in a meat packing house and in certain seafood processing plants. The dry ice pellets in these applications are delivered directly onto the food being processed to rapidly cool the food and to keep the food below a specified maximum temperature to prevent spoilage while processing and prior to refrigerated storage. Also, dry ice has long been the favored refrigerant for ice cream vendors and distributors.
Dry ice is the preferred means of cooling in such applications since it imparts no color, odor, or taste and has no lingering deleterious effect on the food. Dry ice pellets are even mixed directly into certain batters to keep the batters cold and fresh. For this reason, the dry ice must be pure and free of all contaminants.
In its solid state at standard temperature and pressure, CO.sub.2 has a constant and stable temperature of minus 109.33.degree. F., well below that of many common refrigerants such as ammonia, and has a refrigeration value of 246.25 BTUs per pound. Carbon dioxide is normally transported in its liquid state and stored in refrigerated storage tanks at a pressure of 250 psig and a corresponding temperature of -8.degree. F.
Dry ice provides another attractive feature in that it sublimes (transforms from the solid to the gaseous phase directly, without going through the liquid phase) and, therefore, leaves no residue after yielding its refrigeration effect; no post clean-up or removal of residual liquid is required. Furthermore, CO.sub.2 is neither toxic, poisonous, reactive with other chemicals, nor flammable. In fact, CO.sub.2 is commonly used as a fire fighting agent.
When liquid CO.sub.2 is permitted to flash through an expansion device into a chamber (referred to herein as a "snow chamber") at atmospheric pressure from the nominal 250 psig, the liquid flashes to a vapor (referred to herein as revert) and in the process removes the heat of vaporization from the remaining liquid rapidly cooling the liquid to the solid phase in the form of snow. The proportionate amounts of snow and revert depend on the pressure and temperature of the LCO.sub.2 fed to the expansion device. The lower the pressure and temperature of the feed LCO.sub.2, the greater the proportion of snow formed as a result of the free expansion. Rapidly expanding liquid CO.sub.2 at 250 psig and -8.degree. to atmospheric pressure yields about one pound of dry ice as snow and 1.08 pounds of vapor at -109.33.degree. F. This snow may then be compacted through various processes to form blocks or pellets of dry ice.
In order to provide a truly competitive pelletizer system, recovery of revert gas requires compressing and cooling the collected revert to system conditions, commonly 250 psig and -8.degree. F., to recover the CO.sub.2 as liquid feed stock. To reduce the amount of revert produced per pound of feed stock introduced, some pelletizers use a small heat exchanger to pre-chill the LCO.sub.2 below its -8.degree. F. saturation temperature. Revert from the snow chamber may be used to pre-chill feed stock. This technique helps a little, but is an unmanageable situation danger of over cooling and freezing feed stock into a solid mass. Experience has demonstrated that this is an unmanageable situation and, as a result, a small, token heat exchanger is used. Thus, there remains a need for a pelletizer system that recovers 100% of the revert gas that can operate continuously and unattended.
Known dry ice pelletizers, such as that shown in Brooke, U.S. Pat. No. 4,780,119 and assigned to TOMCO2 Equipment Company, commonly use a piston to compress dry ice snow into a block or to compress and extrude the dry ice through a die. Piston devices include piston rings to maintain pressure for compression and such piston devices have several known drawbacks which known pelletizers have been unable to solve. First, piston rings, by their very nature, come into rubbing contact with the cylinder wall surrounding them causing wear and resultant wear products from whatever material the rings are made of, often metal. Metal wear products are unacceptable contaminants in dry ice that will be used in many applications, such as in food processing. Pistons often require some sort of lubricant to prevent piston rings failure and such lubricants also provide an undesirable source of contamination of the dry ice product.
Another problem with known piston operated systems is the hazard of "blow-by" whereby snow blows by the piston rings and accumulates behind the piston. This can ultimately result in rod seal rupture.
While the Brooke machine is the most capable machine known to the inventors, it suffers in that it is a horizontal, hydraulic type pelletizer. It is limited to approximately 600 lb/hr per cylindrical chamber used. (Two chambers have been hydraulically linked to increase the capacity of a skid-mounted unit to 1200 lb/hr. In addition to the contamination previously described, the Brooke machine, unlike the present invention, is unnecessarily complex and is subject to all of the shortcomings inherent in complexity, including cost and parts subject to wear and failure. Also, the Brooke machine only produces pellets as an intermediate product and is intended to produce solid disks of dry ice.
The Brooke machine with its horizontal construction suffers yet another drawback. The hydraulic element (i.e., piston rod) that performs the compaction is under a compressive force during compaction. This means that the hydraulic element is subject to side loading that may cause buckling and consequent misalignment. Further, since the dry ice snow is heavier than the surrounding vapor, the snow falls to the bottom wall of the snow chamber cylinder. Then, when the snow is compacted against the die, it tends to result in an uneven extrusion, denser at the bottom of the die and less dense above the bottom.
Other known pelletizers provide one chamber for the expansion of the LCO.sub.2 and a separate chamber for compaction and extrusion of the pellets. The expansion chamber also separates the revert from the snow. In such machines, snow is produced in an expansion chamber and allowed to fall by gravity into the compaction chamber. While this technique increases the production rate of the pelletizer, production often becomes erratic due to agglomeration of the snow in the expansion chamber. To correct this problem and to keep the snow more consistently flowing into the compaction chamber, known systems have used "rabble" arms or other vibrating devices in cooperation with the expansion chamber. This makes for an expensive and complicated machine with more moving and wearing pans in contact with the snow.
An alternative to an external expansion chamber uses the compaction chamber as a 2-phase separator as well. This puts constraints and demands on the chamber, since it must carry out two sequential unit operations within one chamber and the chamber must have a revert outlet of sufficient capacity that the revert velocity does not entrain and carry out solid snow with it. Snow entrained in the revert can cause plugs, excess chamber pressure, and production stoppage.
Thus, there remains a need for a dry ice pelletizer that is simple in construction and eliminates the possibility of contamination from corrosion and wear products or from lubricants to produce food-grade (FDA) quality pellets consistently and economically. Such a pelletizer should provide marked increased throughput of dry ice product and develop more uniform dry ice pellets. Such a pelletizer should also provide safe and reliable operation for an extended period of time in automatic operation. The pelletizer should provide for the recovery of revert without modifications to the pelletizer and without snow entrainment with the revert which can cause stoppages or inconsistent operation. Such a pelletizer should also provide for portable operation so that the pelletizer can operate at the consumption site, thereby circumventing costly sublimation loss during storage and transportation of the product dry ice.